L15 solution

Embryonic days (E) 13.5–14 embryos from timed breeding were used for transplantation. MGE of embryonic forebrain was dissected and suspended in chilled L15 solution (Gibco, 21083027) with 2% (v/v) of DNase I (Roche, 0471672800). All transplantation and subsequent virus injections were performed on the right hemisphere of the animals. For precise targeting, binocular visual cortex (bV1) of adult transplant recipients was mapped using intrinsic optical imaging (described below) at least one week prior to transplantation. The injection sites around bV1 were thinned using a dental drill (Midwest, 78044) and FG1/4 carbide burr to allow the injection micropipette to penetrate. The tissue was loaded into a beveled glass micropipette (75 µm; Wiretrol 5 µl, Drummond Scientific Company) and injected using a custom-made hydraulic manipulator (Narishige, MO-10). The cells were injected at ~650–700 µm below the surface of the brain. Each transplant recipient received 20–30 nl per injection for a total of six to eight injections, all in the same hemisphere. At the end of the procedure, the scalp was sutured using Perma-Hand Silk (Ethicon, J212H) and the animals were returned to the home cage on a heating pad. Refer to Supplementary Data 1 for the genotype and the age of adult transplant recipients, and the type of donor tissue each received.

VLPs internalization assays were performed using fluorescent M(GFP)NE and M(GFP)NES particles or MN(GFP)ES VLPs. Human pulmonary A549 or A549-hACE2 cells were harvested live in cold PBS 1x (after 24 h in culture on glass coverslip). M(GFP)NE or M(GFP)NES or MN(GFP)ES VLPs were diluted (1:20) in transparent L15 Solution (Gibco) and put into contact with the live cells. The cells were then incubated either at 37 °C for 15 min or at 4 °C for 1 h. Unbound VLPs were removed by washing cells 3 times with cold PBS 1x. The cells were then fixed with 4% PFA and mounted with Prolong Gold antifade reagent with DAPI (Invitrogen) for confocal imaging. Confocal fluorescence images were generated using a CD7 confocal laser-scanning microscope (Zeiss, Germany) equipped with a 50 × 0.5 ×, 1.2 NA oil objective or with a LSM980 confocal laser-scanning microscope (Zeiss, Germany) equipped with a 63 × 1.4 NA oil objective. The cells were imaged as Z stack with 0.3 µm sections and deconvoluted using Deconvolution lab plugin in Fiji with a theoretical PSF. Fiji was also used to count the number of MN(GFP)ES VLP (ie. Fluorescent dots) per cell.
To measure VLP distance from cell center, using Icy, we first performed a Z-projection of stacks, then segmented the cells to establish their center position and subsequently performed cell center/M(GFP)-VLP distance measurements.